1. Field of the Invention
This invention relates to variable transformers of the type having a wire coil wound upon an electromagnetic core and having upon the coil a commutating surface along which a contact brush is movable while in electrical engagement therewith, and more particularly to means for reducing the size of the electromagnetic core required for a given size transformer.
2. Description of the Prior Art
Variable transformers are well known in the art and typically, though not invariably, include a toroidal wire coil which is machine-wound upon a magnetic core of similar toroidal shape. In most such devices, the coil is wound so that there is a single layer of wire at the outer diameter of the toroid and a double layer of wire at the inner diameter of the toroid. Such an arrangement is used to facilitate winding a nearly perfect, sequentially-wound coil so that the commutating surface may be formed on the upper surface of segments of wire forming the single layer, by grinding partially through wire segments in the commutator path. Such coils having fine wire use many times more electromagnetic core material relative to the amount of wire. If multiple layers of wire could be economically wound into a nearly perfect, sequentially-wound coil under the commutating surface, relatively more wire and less electromagnetic core material could be used for a given transformer capacity and would result in a smaller, more economical coil. Heretofore, variable transformer coils wound with more than one layer of wire have proved to be difficult to machine-wind, resulting in high reject rates.